Semiconductor optical waveguides, typically, silicon optical waveguides are highly expected to be a technique which will contribute to higher integration of optical communication devices. In response to such expectation, for example, silicon waveguides having functions of an optical modulator, a photodetector, an optical switch, etc. have been provided, and have found their application in optical communications.
In many cases, a conventional single mode fiber (CSMF) is connected to a semiconductor optical waveguide so as to propagate incident light to enter the semiconductor optical waveguide or light having exited from the semiconductor optical waveguide. However, the CSMF has a mode field diameter of approximately 10 μm whereas the semiconductor optical waveguide has a mode field diameter of approximately 1 μm. Thus, the semiconductor optical waveguide butt-joined to the CSMF causes an excessively large connection loss due to a difference in mode field diameter. Due to such a connection loss, the CSMF and the semiconductor optical waveguide butt-joined to each other cannot be put to practical use.
In light of this, there has been proposed a method of making a spot size converter (SSC) in a semiconductor optical waveguide and connecting a CSMF to the SSC (see Patent Literature 1). However, the SSC causes a large loss in a case where the mode field diameter of the semiconductor optical waveguide is increased by use of the SSC substantially up to the mode field diameter of the CSMF. There has been examined a method including (1) butt-joining one end of a bridge fiber having a mode field diameter of 4 μm to a semiconductor optical waveguide having a mode field diameter increased to 4 μm by use of an SSC, and (2) fusion-splicing, to the other end of the bridge fiber, a CSMF having a mode field diameter of 10 μm.
Employed as such a bridge fiber is an optical fiber that includes a thermally diffused expanded core (TEC), specifically, an optical fiber whose core is expandable through thermal diffusion (see Patent Literatures 2 and 3). This reduces a difference in mode field diameter between the bridge fiber and the CSMF, because the core of the bridge fiber expands during fusion-splicing of the core to the CSMF or subsequent heating of the core. It is accordingly possible to reduce a connection loss to a low level at a fusion-splicing point between the bridge fiber and the CSMF.
Note that the core expands through thermal diffusion, because an updopant (i.e., additive for increasing a refractive index of quartz glass) added to form the core diffuses to surroundings of the core when being heated. Assume that germanium (Ge) is added as the updopant for forming the core. In this case, it has been known that the core can expand at a higher rate to surroundings codoped with germanium, phosphorus (P), and fluorine (F) (see Patent Literature 4).